Astral microtubules are a subpopulation of microtubule

Microtubule

Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...

s, which only exist during and immediately before mitosis

Mitosis

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

. They are defined as any microtubule originating from the centrosome

Centrosome

In cell biology, the centrosome is an organelle that serves as the main microtubule organizing center of the animal cell as well as a regulator of cell-cycle progression. It was discovered by Edouard Van Beneden in 1883...

 which does not connect to a kinetochore

Kinetochore

The kinetochore is the protein structure on chromatids where the spindle fibers attach during cell division to pull sister chromatids apart....

. Astral microtubules develop in the actin skeleton and interact with the cell cortex to aid in spindle orientation. They are organized into radial arrays around the centrosomes. The turn-over rate of this population of microtubules is higher than any other population.

The role of astral microtubules is assisted by dynein

Dynein

Dynein is a motor protein in cells which converts the chemical energy contained in ATP into the mechanical energy of movement. Dynein transports various cellular cargo by "walking" along cytoskeletal microtubules towards the minus-end of the microtubule, which is usually oriented towards the cell...

s specific to this role. These dyneins have their light chains (static portion) attached to the cell membrane, and their globular parts (dynamic portions) attached to the microtubules. The globular chains attempt to move towards the centrosome, but seeing as they are bound to the cell membrane, this results in pulling the centrosomes towards the membrane, thus assisting cytokinesis

Cytokinesis

Cytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...

.

Astral microtubules are not required for the progression of mitosis, but they are required to ensure the fidelity of the process. The functions of astral microtubules can be generally considered as determination of cell geometry. It is absolutely required for correct positioning and orientation of mitotic spindle

Mitotic spindle

In cell biology, the spindle fibers are the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells...

 apparatus. It is also involved in determination of cell division site based on the geometry and polarity of the cells.

The maintenance of astral microtubules is dependent on the integrity of centrosome. It is also dependent on several microtubule-associated proteins such as EB1

MAPRE1

Microtubule-associated protein RP/EB family member 1 is a protein that in humans is encoded by the MAPRE1 gene.-Further reading:...

 and Adenomatous Polyposis Coli (APC).